Optical Field Effect in Silicon

Structure in the reflectance is related to interband transitions at critical points and therefore provides a major source of information in the analysis of the band structure. This paper describes a derivative technique which enhances this structure considerably. The density of states at the critical points is modulated through an ac electric field at the reflecting surface and the resulting change in the reflectance is amplified phase-sensitively. At a photon energy of 3.4 eV, three peaks are resolved in silicon within only 0.1 eV. Their dependence upon temperature and orientation of the reflecting surface suggests an assignment to a transition at the $\Gamma$ point for the peak at 3.34 eV and at the $L$ point for the peak at 3.45 eV, with an excitonic peak in between. Structure in the reflectance response at 4.20 eV is correlated to the $X_4-X_1$ transition. Since the effect is caused by a modulation of the surface potential, it is possible to draw conclusions about the parameters of the surface from the response of the reflectance change to temperature, dc bias, and modulation frequency. An optical technique is outlined which will complement the electrical field-effect technique.